This invention relates to retinal display devices and optical scanner devices, and more particularly to methods for duplicating exit pupils and for switching among exit pupils.
A virtual retinal display (VRD.sub..TM.) device is an optical device for generating a virtual image to be perceived by a viewer's eye. Light is emitted from a light source, collimated through a lens, then passed through a scanning device. The scanning device defines a scanning pattern for the light. The scanned light converges to focus points of an intermediate image plane. As the scanning occurs the focus point moves along the image plane (e.g., in a raster scanning pattern). The light then diverges beyond the plane. An eyepiece is positioned along the light path beyond the intermediate image plane at some desired focal length. An "exit pupil" occurs shortly beyond the eyepiece in an area where a viewer's eye pupil is to be positioned.
A viewer looks into the eyepiece to view an image. The eyepiece receives light that is being deflected along a raster pattern. Light thus impinges on the viewer's eye pupil at differing angles at different times during the scanning cycle. This range of angles determines the size of the virtual image perceived by the viewer. Modulation of the light during the scanning cycle determines the content of the image. For a see-through display a user sees the real world environment around the user, plus the added image of the display projected onto the retina.
Typically, the exit pupil defined by the display device is less than 2 mm in diameter and often less than 1 mm in diameter. The viewer's eye pupil varies from approximately 2 mm in diameter under bright light to approximately 7 mm in a dark environment. Because of the small exit pupil, a first step for a viewer is to adjust eye position to find the exit pupil. The viewer's pupil needs to achieve and maintain alignment with the display device's exit pupil so that light from the display device can enter the user's pupil and reach the viewer's retina. While in alignment, the light can scan directly onto the viewer's retina without any intermediary screens, cathode ray tubes (CRT's) or liquid crystal display devices (LCD's). The result is an image perceived by the viewer.
A shortcoming of conventional scanned retinal displays is the difficulty of maintaining alignment between the exit pupil and the viewer's pupil. If the viewer moves, alignment may be lost. Movement is problematic because the viewer's eye tends to move when the viewer attempts to view a peripheral portion of the image. Movement of the viewer's head relative to the display or even blinking may move the eye relative to the exit pupil. As a result, conventional exit pupils are inconvenient for the viewer. In particular a lay consumer using a virtual retinal display would find the alignment requirement difficult to maintain for long term viewing applications, such as entertainment, or for wide field view images. Accordingly, there is a need for a scanned display device having an exit pupil defined so as to enable easier viewing of the image.
Within the scanned display, optical scanners typically scan light onto the retina. In an exemplary configuration one scanner is used to provide horizontal deflection of a light beam, while another scanner is used to provide vertical deflection of the light beam. Together the two scanners deflect the light beam along a raster or similar pattern. Each of the scanners includes a respective mirror that deflects light along a path defined by the deflection angle of the mirror. At the same time that the beam is scanned, the beam also is modulated responsive to image information, such as video signals. Where the display is a color display, the image information includes RGB color data that is used to separately modulate red, green and blue components of the light beam. The three modulated components are then combined and scanned in raster format onto the retina to produce a color virtual image.
Scanning rate and physical deflection distance characterize the movement of the scanner's mirror. In the context of a scanned retinal display the scanning rate and deflection angles are defined to meet the limits of the human eye. The scanning rate determines the number of times the beam strikes a region of the retina in a given time period. For the eye to continually perceive an ongoing image the light beam rescans the image, or a changing image, in periodic fashion. Analogous to refreshing a pixel on a display screen, the eye's retinal receptors must receive light from the scanning light beam periodically. The minimum refresh rate is a function of the light adaptive ability of the eye, the image luminance, and the length of time the retinal receptors perceive luminance after light impinges. To achieve television quality imaging the refresh rate typically is at least 50 to 60 times per second (i.e., .gtoreq.50 Hz to 60 Hz). Further, to perceive continuous movement within an image the refresh rate typically is at least 30 Hz.
The mirror deflection angle is defined by the desired field of view and the eyepiece magnification. The field of view is the range of angles at which the retina receives light. Larger fields of view correspond to larger scan angles.